Project Summary Anxiety and trauma-related disorders such as post-traumatic stress disorder are common and debilitating mental health disorders, but conventional treatment options are limited in long-term efficacy. Common therapeutic strategies aim to enhance fear extinction, the process by which the fear response to a fear-eliciting cue decays over time. However, fear extinction memories are labile and susceptible to the return of fear (relapse), even following successful extinction. Unfortunately, many of the manipulations that have been reported to enhance fear extinction memory either have not been tested for their ability to reduce fear relapse, or have failed to reliably do so. Identification of novel strategies to prevent fear relapse after extinction is thus of utmost importance to mental health. The nigrostriatal dopamine (NS DA) pathway, consisting of DA neurons in the substantia nigra pars compacta (SNc) projecting to the dorsal medial striatum (DMS), is classically considered in the context of motor behavior, but growing evidence supports a role for NS DA in emotional regulation. Further extending the purview of NS DA functions to include fear extinction learning, we observed that increasing high-frequency (phasic) activity of the NS DA pathway during fear extinction with viral-mediated chemogenetic techniques enhances extinction memory and prevents the return of fear after extinction. Phasic DA release preferentially signals through low-affinity D1 receptors, and our preliminary data suggest that D1 receptors in the DMS mediate the observed effects of NS DA pathway activation. We observed both that 1) neurons expressing D1 receptors in the DMS are recruited during fear extinction learning, and 2) pharmacological activation of DMS D1 receptors during fear extinction strengthens fear extinction memory. These data suggest that the NS DA pathway represents a previously unidentified component of fear extinction; activation of which could strengthen extinction. Harnessing NS DA during extinction could be especially important for females, who have a particularly high prevalence of anxiety, as well as a fear extinction retrieval deficit when fear extinction is learned during phases of the estrus cycle when estrogen is low. Consistent with the idea that phasic activity of NS DA enhances relapse- resistant fear extinction, we observed that voluntary exercise increases phasic DA release in the DMS, strengthens fear extinction, and reduces fear relapse in males. In females, exercise rescues the fear extinction retrieval deficit present when extinction is learned during estrus phases when estrogen is low. The goal of this proposal is to test the hypothesis, supported by our preliminary data, that NS DA represents a novel circuit for the enhancement of relapse-resistant fear extinction in males & females. Using chemogenetic, neurochemical and pharmacological strategies, we aim to determine whether activation of the NS DA pathway & D1 receptor signaling in the DS 1) enhances the learning of relapse-resistant fear extinction & 2) is necessary for the ability of acute exercise to augment fear extinction & prevent relapse.